Quasistellar Objects: Overview ENCYCLOPEDIA of ASTRONOMY and ASTROPHYSICS
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eaa.iop.org DOI: 10.1888/0333750888/1585 Quasistellar Objects: Overview Patrick Osmer From Encyclopedia of Astronomy & Astrophysics P. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888 Institute of Physics Publishing Bristol and Philadelphia Downloaded on Thu Mar 02 22:43:08 GMT 2006 [131.215.103.76] Terms and Conditions Quasistellar Objects: Overview ENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICS Quasistellar Objects: Overview Quasistellar objects, or quasars, were defined originally as star-like objects of large redshift. Quasars are believed to be powered by the accretion of matter onto massive black holes at the centers of galaxies, a process that emits more energy than thermonuclear reactions. Today, quasars are considered to be the most luminous members of the general class of objects called active galactic nuclei, or AGNs. Quasars are the most luminous objects in the universe. This article begins with a brief history of the discovery of quasars. Next it describes their main properties and the concepts that have been developed to explain them. It continues with a description of their nature and theoretical models. Then additional properties and topics are considered: absorption lines, host galaxies, Figure 1. Optical image of 3C273. The object looks stellar in and luminosity functions and evolution. this image except that it is accompanied by a jet of radiation extending to the lower right. (Credit National Optical Astronomy Observatories/National Science Foundation. History Copyright Association of Universities for Research in In 1960 Mathews and Sandage identified the radio source Astronomy Inc (AURA), all rights reserved.) 3C 48 (the 48th object in the 3rd Cambridge catalog of radio sources) with a star-like object of 16th magnitude. by more than a factor of 4. The light from 3C 9 had taken Its optical spectrum showed unusual emission lines that 80% of the age of the universe, or more than 10 billion could not be identified, and the object was considered to years, to reach Earth. The ability to observe objects at be a radio star, more of which were identified in the redshifts greater than 2 meant that astronomers could course of their work. In 1962, Hazard, Mackey and probe back in time to within a few billion years of the big Shimmins established an accurate radio position for bang. another source, the BRIGHT QUASAR 3C 273, by observing The unusual nature of quasars was compounded by the object as it was being occulted by the Moon. Schmidt the work of Smith and Hoffleit, who showed that 3C 273 used this position to identify 3C 273 with a 13th varied significantly in brightness on timescales of magnitude star-like object (figure 1), whose spectrum months. This variability indicated that the characteristic also displayed unusual emission lines. Schmidt size of the emission region was light-months and led subsequently realized that the emission lines could be immediately to the question, how could such a small identified with lines from hydrogen and oxygen object produce more light than an entire galaxy, with a redshifted by 15.8% of their rest wavelength. A redshift typical size of 100 000 light-years? of 0.158 was unprecedented for such a bright object. If To summarize, quasars revolutionized astronomy in the redshift arose from the expansion of the universe, it the 1960s for two reasons: (1) they pushed back the meant that 3C 273 was about two billion light-years limits of the observable universe significantly in both distant, according to the Hubble law, in which the distance and lookback time and (2) their compact size redshift of objects increases in proportion to their and great luminosities could not be explained in terms of distance. Furthermore, the combination of the observed the stars and galaxies known at the time. Quasars opened brightness of 3C 273 with the inferred distance meant new frontiers in cosmology and stimulated the that it had an intrinsic luminosity 100 times that of an development of a new subject—relativistic astrophysics. entire galaxy like the Milky Way. Following Schmidt’s work, it was immediately Main properties and concepts realized that the redshift of 3C 48 was 0.37. By 1965, 3C The main observed properties of quasars are the 9 was found to have a redshift of 2.01, well in excess of following. the 0.46 value for 3C 295, the most distant galaxy known 1. Nuclei that appear starlike in optical images. at the time. It had taken more than 30 yr of work on Extended emission can now often be detected around galaxies with the largest telescopes in the world for the nucleus, and jets extending away from the astronomers to push the limit of the observable universe nucleus are occasionally seen. to the distance of 3C 295. Only 2 yr after the discovery of 2. Spectra showing broad emission lines with widths quasars, the maximum observed redshift had increased greater than observed in normal galaxies and a range Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK 1 Quasistellar Objects: Overview ENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICS of ionization broader than seen in typical nebulae (see BL LACERTAE, BL LACERTAE OBJECTS and AGN: ionized by stars. VARIABILITY). 3. Structures in quasars with observable radio emission Variability provides powerful tools for determining that range in angular extent from m arcsec to tens of the inner structure and nature of quasars. When the arcsec. The most compact structures are often central source itself varies in brightness, the effects of the observed to expand on timescales of years. variation propagate outward with time and produce 4. Radiation of similar power at energies ranging from changes in, for example, the emission line spectra and γ-rays to the far-infrared (100 µm) region of the continuum radiation from the material surrounding the spectrum, with a decrease in energy at radio central source. In the case of blazars, we are seeing frequencies. almost directly down the axis of the jets of material that 5. Redshifts, z, of spectral features ranging from 0.1 to are being ejected from the central regions. By following 5. how the radiation from the jets varies at different 6. Variability in brightness at different wavelengths on wavelengths with time, we are able to infer the physical timescales as short as days to weeks. conditions and processes that are occurring in the jets. 7. Luminosities as high as 1014 Suns. Observations of the variability of quasars carried out 8. Total energy in high-energy electrons in the region at different wavelengths in intensive campaigns and also of radio emission that can exceed 1060 erg. in programs extending for years have provided some of Additional information about these properties and the the most direct information on the nature of the inner concepts they imply about the nature of quasars follows regions in quasars and AGNs. These programs have below. shown that the emission line regions are considerably smaller and closer to the central source than was Redshifts originally thought. The large redshifts of quasars are one of their distinctive features, and the value of the greatest known redshift, z, Continuum emission and multi-wavelength continued to increase after 1965, surpassing z = 3 in 1973 observations and z = 4 in 1987. The maximum value reached z = 4.9 in Quasars emit radiation strongly in bands ranging from γ- 1991, and an object at z = 5 was discovered in the first rays and x-rays to the far infrared (100 µm). The amount stages of the Sloan Digital Sky Survey in 1998 (see of energy emitted in each band by most objects is QUASISTELLAR OBJECTS: SURVEYS). The great majority of remarkably similar, in contrast to normal thermal quasars, however, have redshifts less than 2.5. This radiation from stars, which is much more peaked and occurs for two reasons: (1) such quasars are easier to find restricted in wavelength. For example, most stars are cool because they are brighter in the ultraviolet than most stars and are faint at ultraviolet wavelengths. In contrast, most and (2) they are intrinsically more numerous than quasars quasars with redshifts less than 2.5 are bright at with z > 2.5. The low-redshift limit of quasars is ultraviolet wavelengths, a property that is very helpful in normally set at 0.1 because objects at lower redshift are distinguishing them from the more numerous stars that usually resolvable as galaxies and are cataloged as are found in sky surveys. ACTIVE GALACTIC NUCLEI. Although quasars were discovered through their In addition to providing our first view of the radio emission, about 90% of quasars do not emit universe at z > 0.5, quasars also proved to be valuable strongly at radio wavelengths and are classified as radio cosmological probes because they were luminous enough quiet. Radio-loud quasars are typically 100 times brighter to yield clues on the properties of matter along the line of at radio wavelengths than radio-quiet quasars. sight back to the Earth (see discussion on absorption lines The continuum emission in quasars appears to arise below and in more detail in QUASISTELLAR OBJECTS: from a combination of thermal and non-thermal INTERVENING ABSORPTION LINES). processes. For the latter, synchrotron emission from relativistic electrons produces the lower-energy radiation. Variability The x-ray and γ-ray emission may come from the inverse Observations indicate that variability is a common Compton scattering of lower-energy photons by high- feature of quasars and most quasars are believed to show energy electrons, although thermal mechanisms may also variations in light at the 10–40% level over timescales of play a part in x-ray emission.